The solid-state imaging element such as the CCD imaging sensor or the CMOS imaging sensor is mounted on an imaging device such as a digital video camera or a digital still camera, and is mounted on various kinds of electronic devices each having an imaging function, such as a camera cell-phone, a scanner, a copying machine, and a fax machine.
The solid-state imaging element includes a photoelectric conversion unit such as a photodiode in a substrate to generate electric charges by photoelectrically converting light inputted to the substrate. The generated electric charges are accumulated in an accumulation region in the substrate, and are subsequently transferred to a read-out region in the substrate through a transfer section. Thus, based on the charges transferred to the read-out region, one signal composing an image is generated.
Recently, it has been required to make the solid-state imaging element highly sensitive. However, when the accumulation region increases in size so that the solid-state imaging element is made highly sensitive, there is a decrease in transfer speed of the electric charges through the transfer section, which causes a problem.
This problem will be described with reference to FIG. 21. FIG. 21 is a view showing a conventional solid-state imaging element. In addition, FIG. 21 (a) is a plan view of one pixel in the solid-state imaging element, FIG. 21 (b) is a cross-sectional view showing a cross-sectional surface taken along X-X in FIG. 21 (a). Furthermore, FIG. 21 (c) is a graph showing a potential in the cross-sectional surface taken along X-X in FIG. 21 (a).
As shown in FIGS. 21 (a) and (b), a solid-state imaging element 100 includes a substrate 101, an accumulation region 102 formed in the substrate 101, for accumulating electrons generated by a photoelectric conversion, a read-out region 103 formed in the substrate 101, for receiving transferred electrons accumulated in the accumulation region 102, a transfer section 104 for transferring the electrons from the accumulation region 102 to the read-out region 103, and an insulating film 105 formed on a surface of the substrate 101. The transfer section 104 serves as a gate electrode formed on the insulating film 105 and is formed between the accumulation region 102 and the read-out region 103.
The substrate 101 has a P type (P-sub), the accumulation region 102 has an N type (N−), and the read-out region 103 has the N type (N+). In the solid-state imaging element 100, an N-type high-concentration impurity region 1021 (impurity concentration modulation region) having an N type (N) is formed by separately implanting an N-type impurity into an implantation region 106 which is provided in the accumulation region 102 and which is provided adjacent to the transfer section 104. Therefore, according to the solid-state imaging element 100 in the present example, a photodiode is composed of the substrate 101 and the accumulation region 102, and the electrons are accumulated in the accumulation region 102.
When a predetermined potential is applied to the transfer section 104 in this solid-state imaging element 100, the potential in the substrate 101 just below the transfer section 104 is lowered, and the electrons accumulated in the accumulation region 102 are transferred to the read-out region 103. At this time, when an area of the accumulation region 102 is large as descried above, some electrons are accumulated in a position far away from the transfer section 104, in the accumulation region 102. Thus, it takes a long time for the electrons to reach the transfer section 104.
When the N-type high-concentration impurity region 1021 is provided in the accumulation region 102, in the solid-state imaging element 100, the electrons are accumulated in the accumulation region 102. However, as shown in FIG. 21 (c), a potential in the N-type high-concentration impurity region 1021 is lower than that of a peripheral part due to the implantation of the N-type impurity, but the potential is flat. Therefore, movement of the electrons accumulated in the N-type high-concentration impurity region 1021 to the transfer section 104 is not particularly accelerated, and it takes a long time for the electrons to reach the transfer section 104.
Thus, in the case where the electrons accumulated in the accumulation region 102 cannot be completely transferred to the read-out region 103 within a predetermined read-out period, the electrons remain in the accumulation region 102, and these electrons are added to electrons to be generated by next photoelectric conversion, so that a residual image is generated in an obtained image, which is the problem.
Thus, for example, Patent Document 1 discloses a solid-state imaging element in which movement of the electrons to the transfer section is accelerated by inclining a potential in the accumulation region. This solid-state imaging element will be described with reference to FIG. 22. FIG. 22 is a view showing a conventional solid-state imaging element. In addition, FIG. 22 (a) is a plan view of one pixel in the solid-state imaging element, FIG. 22 (b) is a cross-sectional view showing a cross-sectional surface taken along Y-Y in FIG. 22 (a). Furthermore, FIG. 22 (c) is a graph showing a potential in the cross-sectional surface taken along Y-Y in FIG. 22 (a).
As shown in FIGS. 22 (a) and (b), a solid-state imaging element 200 includes a substrate 201, accumulation regions 2021 to 2024 which are formed in the substrate 201 and which accumulate electrons generated by a photoelectric conversion, a read-out region 203 which are formed in the substrate 201 and which receives the transferred electrons accumulated in the accumulation regions 2021 to 2024, a transfer section 204 for transferring the electrons from the accumulation region 2024 to the read-out region 203, and an insulating film 205 formed on a surface of the substrate 201. The transfer section 204 serves as a gate electrode formed on the insulating film 205 and is formed between the accumulation region 2024 and the read-out region 203.
The substrate 201 has the P type (P-sub), the accumulation regions 2021 to 2024 have the N type, and the read-out region 203 has the N type (N+). Therefore, according to the solid-state imaging element 200 in the present example, a photodiode is formed of the substrate 201 and the accumulation regions 2021 to 2024, and the electrons are accumulated in the accumulation regions 2021 to 2024. The accumulation regions 2021 to 2024 are formed by sequentially implanting the N-type impurity to implantation regions 2051 to 2054. In addition, the implantation regions 2051 to 2054 are close to the transfer section 204, respectively, and the implantation regions 2051, 2052, 2053, and 2054 are decreased in size in this order.
According to the solid-state imaging element 200, a concentration (N---) of the N-type impurity in the accumulation region 2021 provided farthest from the transfer section 204 is lowest, a concentration (N--) of the N-type impurity in the accumulation region 2022 provided second farthest is second lowest, a concentration (N-) of the N-type impurity in the accumulation region 2023 provided third farthest is third lowest, and a concentration (N) of the N-type impurity in the accumulation region 2024 provided closest to the transfer section 204 is highest. Therefore, as shown in FIG. 22 (c), the potential in the accumulation regions 2021 to 2024 can be inclined so as to decrease with the decreasing distance to the transfer section 204. Thus, it is possible to accelerate the movement of the electrons in the accumulation regions 2021 to 2024 to the transfer section 204. Therefore, even when the area of the accumulation regions 2021 to 2024 increases, the electrons accumulated in the accumulation regions 2021 to 2024 can be immediately transferred to the read-out region 203.